Quantifying the Effects of Dam Removal on the Structure and Function of Fish Assemblages in the Penobscot River

The Penobscot River drains the largest watershed in Maine, and once provided spawning and juvenile rearing habitats for migratory fish prior to dam construction in the 1800s. The removal of two main-stem dams and improved upstream fish passage at a third dam (between 2011 and 2014) should increase passage success for anadromous and resident fishes, and thus fish assemblages should change. Quantitative assessment of the effects of dam removal on fish assemblages in large rivers is inadequate due to a lack of comparative pre- and post-removal data. The purpose of this study is to quantify fish assemblages before, during, and after dam removal. Boat electrofishing data were collected in 2010 during the early summer and again in the fall using standardized methods along 1000 meter transects that were established during a pilot study. Additional 500 meter transects (n=44) were selected and surveyed using a random-stratified sampling design. All main-stem Penobscot River surveys were conducted within ~45 kilometers of river, which includes tidal zones, impoundments, and free flowing reaches upstream to the fourth main-stem dam. In total, 29 species were captured in the Penobscot River in 2010: Seven are diadromous, and 22 are resident. Only two anadromous species, Atlantic salmon (Salmo salar) and sea lamprey (Petromyzon marinus) were captured above the furthest downstream dam (Veazie Dam), while most anadromous species were restricted to the tidal portions of the river. Above Veazie Dam, the total fish biomass was dominated by white sucker and non-native smallmouth bass. Patterns in biomass, abundance, and species composition suggest that main-stem dams interrupt longitudinal transitions among fish assemblages in the Penobscot River. These patterns should change as a result of dam removal.